Not too long ago I opted to build a footswitch for my Peavey Valveking instead of coughing up $45 for the factory model. No special electronics involved, just switches, LEDs, a 1/4" stereo jack, 1/4" stereo cable, and an aluminum project box from Radio Shack.
Then I noticed a lot of other amps have a footswitch that turns the reverb on/off in lieu of the gain boost that the Valveking has, and then I wanted to have that option too. So I built a switch just for that. Like the other footswitch it's real simple; a switch, RCA jacks, and a plastic project box from Radio Shack. Unfortunately a LED wasn't in the plan since it would require a battery to work.
Building these prove to be a nice escape from life and let me work on my painting skills. Eventually I'll order a proper enclosure and put everything in one box.